Once fusion has been initiated, the star balances the pressure of its outer layers with the energy generated within and achieves hydrostatic equilibrium. At this point, it is considered to have joined the main sequence. The best way to explain the main sequence is visually. If you look at thie chart below (called a Hertzprung-Russell or H-R diagram in honor of its creators) of stellar mass versus luminosity, you’ll notice a densely-populated band where most of the stars are located. Within this band, the luminosity of a star is dependent on its mass. While a star is “burning” hydrogen, this is where it resides. (Keep in mind that the mass of a star is like the fuel rods in a fission reactor, controlling the rate of the reaction. The larger the star, the faster the fusion process occurs and the more rapidly a star depletes its fuel.) Large stars don’t spend very long on the main sequence, while small ones may spend tens of billions of years on it.
The mass of a star affects its core temperature (and its color, which is directly proportional to its temperature), which in turn determines its predominant fusion process and its structure. Very hot stars, like a pot of boiling water, have exceedingly strong convection currents that transport both energy and matter throughout the star’s interior, while smaller stars may have little or no convection occurring. Convection not only affects how effectively energy makes it to the surface - it also determines how well fresh fuel gets distributed within the star, which may subsequently affect how the next phase in a star’s life proceeds.
